Oxidation of organic compounds is of significant concern in many areas, including waste degradation, the manufacture of wood products, and the preparation of adhesive materials. Accordingly, significant effort has been expended to develop methods of oxidizing organic compounds. Low-cost, environmentally-friendly systems to effectively oxidize organic compounds are being sought. One approach has focused on the use of free-radicals. Free-radicals are typically highly energetic and unstable, and can oxidize a broad spectrum of compounds.
The generation of free-radicals by biological systems is known. Research by Koenigs on the action of “Fenton” reagents against cellulose and wood cell wall components suggested that iron and hydrogen peroxide were involved in the production of highly reactive hydroxyl radicals which could initiate the depolymerization of cellulose in wood as: Fe2++H2O2→Fe3++HO.+HO— (“Hydrogen peroxide and iron: a microbial cellulolytic system?” in Cellulose as a Chemical and Energy Resource. Symposium 5, Biotechnology and Bioengineering. Wilke, C. R. (Ed.), John Wiley and Sons, New York 5:151-159 (1975)). Hydroxyl radicals can also be generated in the presence of metals by the related Haber-Weiss reaction (Haber and Weiss, Proc. R. Soc., 147:332-351 (1934)) as: O2—+H2O2→O2+HO—+HO. This reaction is now known to occur via the superoxide reduction of iron with hydrogen peroxide oxidation of iron to produce hydroxyl radicals.
One potential application of free radical oxidation is the production of wood products. Lignin comprises as much as 40% of wood's mass. Lignin is composed of phenylpropane units linked by several forms of ether or carbon-carbon bonds (Sjöström and Eero, Wood Chemistry, Academic Press, Inc., San Diego, Calif., pp. 12, 13, 73-84 (1993)). Lignin's random and noncrystalline network structure makes it a very thermodynamically stable biopolymer (Glasser, W. G., Forest Prod. J., 31(3):24-29 (1981)). The biosynthesis of lignin begins with phenylpropanoid precursors through an enzymatic oxidative coupling mechanism between free phenoxy radicals.
Bonding of lignocellulosic material is essential for the manufacture of a variety of wood products including particleboard, fiberboard, oriented strand board, laminated wood products, and plywood. In current commercial wood bonding processes, an adhesive is spread or sprayed on the surface of the material. The theories for adhesion between the adhesive and wood structure or its components include mechanical interlocking, diffusion, and adsorption or surface reaction (Pocius, A. V., Adhesion and Adhesives Technology: An introduction, Carl Hanser Verlag, Munich, Germany, pp. 118-130 (1997)). Procedures have been proposed to create such adhesion through formation of wood-to-wood chemical bonds, but have not met commercial acceptance. Non-enzymatic methods to promote autoadhesion between lignocellulosic materials have received much attention over the last century. Linzell et al. (U.S. Pat. No. 2,388,487 (1945)) patented the fabrication of compressed fiber composites through the use of a ferric compound, such as ferric sulfate, to facilitate a self-bonding mechanism of wood. Stofko et al. (U.S. Pat. No. 4,007,312 (1977)) patented a process for bonding lignocellulosic materials through the use of a liquid carrier and an oxidant with heat and pressure. In their system, white fir wood shavings with a moisture content of about 12% were sprayed with a solution containing hydrogen peroxide while another equal part of the shavings were sprayed with a catalyst solution of ferrous sulfate and hydrochloric acid. Then, equal amounts of the respective shavings sprayed with the reactants were mixed together and a conventional particle mat was formed. The mat was cold-pressed, hot-pressed, and then samples were tested for internal bond strength. The strength of the bond was comparable to the strength achieved by traditional adhesives.
Enzymatic systems to promote lignin activation and fiber bonding have also been proposed. For example, Kharazipour et al., J. Adhesion Sci Tech. 11(3): pp. 419-4217 (1997) and others (Haars, A. and Hutterman, A., U.S. Pat. No. 4,432,921 (1984); Felby et al., Appl. Microbiol., Biotechnol., 48:459-464 (1997)) described a procedure for bonding wood fragments together in the manufacturing of a composite product. A commonality in their procedures for bonding wood fragments is the activation of the middle lamella lignin of the wood cell wall through incubation with phenol-oxidizing enzymes. Using this technique, molded products were created without additional bonding agents or chemicals. The chemical reactions involved in these self-bonding systems are not fully understood, but oxidative coupling of phenolic units contained in wood is either the main or at least one of the main reactions leading to autoadhesion of lignocellulosic materials (Stofko, J. and Zavarin, E., U.S. Pat. No. 4,007,312 (1977); Haars, A. and Hutterman, A., U.S. Pat. No. 4,432,921 (1984); Kharazipour et al., J. Adhesion Sci. Technol., 11(3):419-427 (1997); Felby et al., Appl. Microbiol. Biotechnol., 48:459-464 (1997)). Possibly, phenolic free radical formation and subsequent coupling occurs at precisely the time surfaces to be bonded are in close contact (Stofko, J. and Zavarin, E., U.S. Pat. No. 4,007,312 (1977)) (some polysaccharide-to-polysaccharide or lignin-to-polysaccharide bonding may also occur during the oxidation).
Certain brown-rot fungi, such as Gloeophyllum trabeum, secrete low molecular weight compounds, initially described as catecholate phenolics, that have been hypothesized to be involved in the degradation of wood (Koenigs, J. W., Arch. Microbio., 99:129-145 (1974); Backa et al., Holzforschung, 46(1):61-67 (1992); Hyde et al., Microbiology, 143:259-266 (1997); Hirano et al., Mokuzai Gakkaishi, 41:334-341 (1995); Jellison et al., App. Microbiol. Biotechnol., 35:805-809 (1991); Chandhoke et al., FEMS Microbiol. Lett., 90:236-266 (1992); Goodell et al., J. Biotech., 53:133-1 62 (1997); Paszczynski et al., 1999; Kerem et al., 1999; Xu et al., J. Biotechnology, 67:43-57 (2001)). The orthodihydroxy forms of these catecholate compounds have the capability to bind and reduce the oxidized (ferric) form of iron (Pracht et al., “Abiotic Fe(III) Induced Mineralization of Phenolic Substances”, Chemosphera, in press (2001)). The reduced iron is then available to participate in Fenton reactions with hydrogen peroxide produced by wood-rotting fungi (Koenigs, J. W., Arch. Microbio., 99:129-145 (1974); Hyde et al., Microbiology, 143:259-266 (1997); (Haber et al., Proc. R. Soc. London, 147:332-351 (1934); Schimdt et al., Am. Wood Preserver Assoc., 77:157-164 (1981)). The resulting highly reactive hydroxyl radicals may then initiate degradation of the wood cell wall through phenolic oxidation. Therefore, the chelators produced by the fungi likely play a role in non-enzymatic wood decay processes since enzymes have been shown to be too large for initial wood cell wall penetration (Flournoy et al., Holzforschung, 45: 383-388 (1991)).
Another potential application of free radical oxidation is in the preparation of adhesive materials. It is known that enzymatically oxidized lignin compounds may be used as wood adhesives (Viikari, et al., U.S. Pat. No. 6,287,708). More specifically, it has been postulated that the phenoxy radicals of lignocellulosic materials may provide adhesion to non-oxidized lignocellulosic materials. Components derived from annual plant materials, such as feruloylarabinoxylans, as well as oxidized phenolic polysaccharides can also be used as adhesives for lignocellulosic materials (Feldman et al., WO 96/03546).
Yet another potential application of free radical oxidation is in the degradation of waste products from leaking storage facilities or accidental or even purposeful discharge. The most common types of contaminants found at waste sites are aromatic and aliphatic organic compounds. Organic waste products may be derived from a wide variety of activities, including the manufacture of petroleum products, plastics, and wood products.
Aromatic and aliphatic organic compounds may be present in surface water as well as soil and groundwater. Especially problematic in groundwater and soil contamination are aromatic and aliphatic compounds refined from petroleum hydrocarbons such as gasoline, fuel oil, motor oil, polychlorinated biphenyl (PCB), benzene, toluene, ethyl benzene and xylene as well as organic monomeric waste compounds from the manufacture of plastics. Aromatic and aliphatic waste compounds include halogenated organic substances and solvents which may present a significant carcinogenic risk.
In-situ groundwater and soil remediative techniques using strong oxidizing agents, such as hydrogen peroxide are known. (Vigneri et al., U.S. Pat. No. 5,268,141; Wilson et al., U.S. Pat. No. 5,525,008; Peter et al., U.S. Pat. No. 5,356,539). Although these techniques are partially effective in degrading aromatic and aliphatic organic compounds are waste products, the oxidative techniques were limited because of the short half life of the hydroxyl radical intermediates.
Aromatic organic waste may also be produced, for example, from wood preservation treatment to produce products such as railway ties, telephone poles and marine pilings. Wood is treated with compounds such as creosote or chlorinated phenols such as pentachlorophenol. Degradation of aromatic organic waste from wood preservation treatments using ferrous iron salt and hydrogen peroxide is known. (Eisenhauer H. R., Water Pollution Control Federation Journal, 36(9): 1116-1128 (1964); Jasim et al., U.S. Pat. No. 5,716,528). However, the utility of these methods is limited by the short half life of the hydroxyl radicals.
Aromatic waste is also produced from the use of dyes. Dyes have been used increasingly in the textile and paper industry because of their cost effectiveness, stability and color variety. Currently, there are about 3,000 different dyes available on the commercial market. Among them, azo dyes are the largest class of dyes used in the industry. Other synthesized dyes include anthraquinone, triphenylmethane, and sulfur dyes etc. (Lubs, The Chemistry of Synthetic Dyes and Pigments, Hafner Publishing Co., Darien, Conn. (1970)). Although these dyes help make our world more colorful, the pollution problem caused by their release into the environment has received considerable attention.
To many textile and paper finishing plants, removing color from industrial effluents is a major issue in wastewater treatment. Biological treatment is a commonly used method, and biodegradation of dyes by anaerobic and aerobic microorganisms has been studied extensively during the past several decades (Chung et al., Crit. Rev. Microbiol., 18:175-190 (1992)). Some commonly used microorganisms in dye biodegradation include bacteria, actinomycetes, yeasts, and fungi (Azmi et al., Enzyme and Microbial Technology, 22(2): 185-191 (1998); Banat et al., Bioresource Technology, 58: 217-227 (1997); Paszczynski et al., Enzyme and Microbial Technology, 13(5): 378-384 (1991); Wong et al., Wat. Res., 30(7): 1736-1744 (1996)). Recently, many researchers have also concentrated on enzymatic systems responsible for the degradation and degradation of dyes during biotreatment (Cao et al., Enzyme and Microbial Technology, 15: 810-817 (1993); Young et al., Wat. Res., 31(5): 1187-1193 (1997); Palma et al., Symposium of 7th International Conference on Biotechnology in the Pulp and Paper Industry, Vancouver, B103-105 (1998)). For the more resistant dyes, however, costly physical and/or chemical decolorizing processes are often the only available treatment alternatives. Physical and chemical techniques which have been explored for dye degradation include flocculation combined with flotation, electroflotation, membrane-filtration, ion-exchange, irradiation, precipitation, and adsorption etc. (Lin et al., Wat. Res., 27: 1743-1748 (1993); Ulker et al., J. Environ. Sci. Health, A29: 1-16 (1994); Banat et al., (1996); Huang et al., Am. Dyestuff Reporter, 83: 15-18 (1994); Adams et al., Ozone Sci. Engng., 17: 149-162 (1995)). Although these physical and/or chemical techniques have been shown to be effective with some specific dyes, they have significant shortcomings. Major disadvantages include: costly equipment requirements and operation expenses; large amounts of sludge generated in certain processes; excess amount of chemical usage; low efficiency color reduction; and sensitivity to variable input streams (Banat et al., 1996).
Other approaches to the degradation of dyes involve chemical oxidation processes to remove color. Oxidative techniques are usually found in the literature to treat colored wastewater. Some commonly used chemical oxidants include chlorine and/or ozone (Namboodri et al., American Dyestuff Reporter, 3: 17-22 (1994); Namboodri et al., American Dyestuff Reporter, 4: 17-26 (1994); Perkins et al., Textile Chemist and Colorist, 27(1): 31-37 (1995); Strickland et al., Textile Chemist and Colorist, 27(5): 11-15, (1995)), UV irradiation with H2O2 (Hosono et al., Appl. Radiat. Isot., 44(9): 1199-1203 (1993); Safarzedeh et al., U.S. Pat. No. 5,266,214 (1993); Yang et al., Textile Chemist and Colorist, 30(4): 27-35 (1998)), Fenton's reagent (Spadaro et al., Environ. Sci. Technol., 28(7): 1389-1393 (1994); Zhu et al., Wat. Res., 30(12): 2949-2954 (1996); Nakagawa, et al., Biol. Pharm. Bull., 16(11): 1061-1064 (1993)) and combinations of these activators. Methods of degrading organic species via oxidative processes are known in the art. See Table 1.
Previous studies have shown that active oxygen species may play a major role in many or most dye oxidation processes. For some mills, certain oxidation treatment schemes may be applicable. None of these processes, however, is effective enough to be used commonly for mills or dye mixtures. In addition, none of these methods use a redox cycling chelator.
Free radicals, especially oxygen based radical species, are very active and strong oxidants that are capable of breaking down dye molecules. Nakagawa et al. studied the bleaching profiles of cyanine dyes exposed to a controlled Fenton reaction. They found under different conditions, hydroxyl radicals (.OH) and/or superoxide radicals (.O2−) were the primary radical responsible for the bleaching of cyanine dyes. However, these free radicals, especially the hydroxyl radical, usually have short lifetimes that limited their application. Safarzedeh described a mediated Fenton method, which employed the photolysis of ferric oxalate to keep generating Fe(II) for Fenton's reaction in the treatment of organic contaminants. The method, however, has some drawbacks such as the competitive UV absorption by byproducts, which reduced the efficiency of treatment and limited the concentration of contaminants that can be treated.
TABLE 1Comparison of degradations based on active oxygen speciesPROCESSAdvantagesDisadvantagesNaOClSimple equipment and processHigh toxicityrapid degradationSalt formationO3Short reaction timesHigh equipment costNo salt and sludge formationNot applicable for all dye typesNo COD reductionToxicity and hazard handlingUV/H2O2Short reaction timeNot applicable for all dye typesReduction of CODRelatively high energy andNo salt and sludge formationequipment costLimited productionFentonSimple equipment and easyLong reaction timeoperationSalt and sludge formationReduction of COD (exceptwith reactive dyes)Increase of DO (dissolvedoxygen) in waterFSR*Simple equipment and easySalt formationoperationGas formation during electrolysisReduction of COD (exceptwith reactive dyes)Increase of DO (dissolvedoxygen) in water*FSR: Fenton Sludge Recycling System is an oxidative degradation process applying Fenton reaction and a subsequent sludge recycling stage.
Therefore, there is a need to develop a novel, effective oxidation process for the degradation of aromatic and aliphatic organic compounds. In addition, there is a need in the art for a non enzymatic, chelator-mediated, free radical system (Goodell et al., J. Biotech., 53: 133-1 62 (1997); Xu et al., J. Biotech., 67, 43-57 (2001)) for oxidation of lignocellulosic material and formation of wood fiber composites, which promote a more environmentally friendly and less expensive alternative for bonding of wood in the wood composite industry. Finally, there is an additional need to develop a non-enzymatic method of producing an adhesive formulation by forming phenoxy radicals. Such methods for producing adhesives would be less expensive and more environmentally friendly than the alternative methods for producing adhesives from organic molecules. Surprisingly, the present invention meets these and other needs.